I've signed up for the One-to-One Training offered at the MacStore, as we're an Apple Organisation, well worth the modest investment. My trainer told me about a little App called Midomi which allows you to search for music by humming or singing a few bars, or by mis-spelling the band or song title, or by "grabbing" a sample of what the radio's playing. From there, you can play an audio file or YouTube file, or buy the song from iTunes. It's free and I've had heaps of fun with it. It is actually useful when people say, "Can you get me the music to that song, that song that goes da-da-daaa-da-da?" If you sing really badly, it won't find the track you intended, but you'll laugh a lot at the suggested songs.
I also have "Pianist" by Moocow Music. So I always have a keyboard with me in my phone. That cost a princely AUD$8. There is a Guitar version too.
Who knew life would be better with these toys? They make useful bribes for boisterous young students - cooperative behaviour earns the right to play with my phone.